Short-chain fatty acids (SCFAs) such as acetate and propionate are used as sources of carbon and energy by prokaryotes occupying diverse habitats such as soil, where acetate and propionate are the most abundant fatty acids, or the gastrointestinal tract of humans where the concentration of acetate and propionate can reach high levels. All catabolic pathways for acetate and propionate require these SCFAs to be activated into their corresponding SCFAcyl-CoA forms before they can be converted into metabolites that can enter central metabolism. Acetyl-CoA feeds directly into the TCA cycle, whereas propionyl-CoA can be catabolized via a number of different pathways that convert it into pyruvate, acetate or succinyl-CoA, which then enter the TCA cycle. Thus the regulation of the enzymes which control the conversion of acetate to acetyl-CoA and propionate to propionyl-CoA regulates the entry, in turn, of these basic feedstock molecules into the metabolic systems of many organisms. For example, acetyl CoA and propionyl-CoA are essential precursors of antibiotic production in some organisms.
In enteric bacteria such as Escherichia coli and Salmonella enterica, acetate is enzymatically converted into acetyl-CoA via either one of two pathways. The first pathway requires the involvement of the acetate kinase (AckA, EC 2.7.2.1) and phosphotransacetylase (Pta, EC 2.3.1.8) enzymes. In these bacteria, the enzymes AckA and Pta are responsible for the synthesis of acetyl-CoA when acetate is present in high concentrations in the environment (≧30 mM acetate). This pathway is considered to be the low-affinity pathway for acetate activation. The second pathway for the activation of acetate requires the activity of the enzyme known as ATP-dependent acetate:CoA ligase (AMP forming; EC 6.2.1.1; aka acetyl-CoA synthetase) encoded by the acs gene. Acs is required when the concentration of acetate in the environment is low (≦10 mM acetate), and thus this pathway is considered to be the high-affinity, or principle, pathway for acetate activation in the bacteria. In S. enterica propionate can be converted enzymatically to propionyl-CoA by the ATP-dependent propionate:CoA ligase (AMP forming, EC 6.2.1.17; aka propionyl-CoA synthetase) encoded by the prpE gene, which is a part of the prpBCDE operon. The prpBCDE operon of this bacterium encodes several of the functions needed for the catabolism of propionate. In addition, S. enterica has two distinct propionate kinases (PduW, TdcD), but the genes encoding these enzymes (pduW, tdcD) are part of the propanediol utilization (pduABCDEGHJKLMNOPQSTUVWX) and threonine decarboxylation (tdcBCDEG) operons whose expression is induced only under specific growth conditions. Hence, under conditions where the pduW and tdcD genes are not expressed, propionate activation to propionyl-CoA occurs only via the high-affinity propionyl-CoA synthetase-dependent pathway based on the enzyme encoded by the prpE gene.
There is significant interest in understanding the functioning of these enzymes and understanding how the activity of the enzymes can be regulated. The use of bacteria to produce biomolecules of interest requires that the feedstock molecules be produced in abundance. Yet the mechanisms which control the regulation of these enzymes in their native hosts are obscure. Currently, it is not known how the enzymatic activities of acetyl-CoA synthetase and propionyl-CoA synthetase are regulated. Understanding how the activity of these enzymes are regulated permits the intelligent design of modified bacteria which can accumulate feedstocks for useful reactions and metabolite accumulation in a manner not possible before.